Formation of the endodermal germ layer is a critical step in the initiation of vertebrate embryogenesis. Endodermal lineages form the epithelial lining of the digestive and respiratory tracts, and contribute to the digestive organs. Studies in Xenopus laevis have identified maternal and zygotic genes that establish the endodermal germ layer at the gastrula stage. VegT, a maternal transcription factor, and Nodal-related members of the TGFbeta family are required for endodermal development and the expression of endodermal transcription factors. VegT and Nodal-related genes are also required for the formation of the mesodermal germ layer, a lineage that is functionally and spatially distinct from the endoderm. How VegT and Nodal genes function to establish both the endodermal and mesodermal germ layers is a fundamental, unanswered question in vertebrate development.To better understand the initiation of endodermal development, we have examined the regulation and function of Sox17, an HMG-box transcription factor expressed in the vegetal endoderm domain. The onset of Sox17 expression at the midblastula transition is activated by VegT and the maintenance of Sox17 expression in the gastrula is dependent on Nodal function. Preliminary studies suggest that the early expression of Sox17 in vegetal cells prevents the induction of mesodermal genes by Nodal signals in the vegetal domain. Using molecular and embryological approaches, the following hypothesis will be tested: VegT activation of Sox17 in vegetal cells defines the vegetal endodermal domain by preventing the activation of mesodermal genes in response to Nodal signals. We propose to: 1) Determine the transcriptional mechanisms regulating Sox17 and Mixer by identifying direct activators, determining their time of action and identifying cis-response elements; 2) Define the functional domains of Soxl7 required to inhibit mesodermal gene expression and the mechanisms of interaction with the Nodal pathway; and 3) Identify the signaling pathways and cis-acting elements that prevent Sox17 and Mixer expression outside of the vegetal domain. These studies will elucidate the signaling and transcriptional network that establishes the endodermal germ layer, a process essential for the formation of the functional digestive tract.The proposed studies will elucidate the molecular mechanisms that establish the endodermal germ layer. Given that defects in endoderm formation and function are implicated in embryonic malformations and pathologies of childhood and the adult, these studies may shed light on congenital abnormalities and disease states. Furthermore, this focus on mechanisms of endodermal determination may have an impact on advances in endodermal stem cells, tissue regeneration and organ culture.